This invention relates generally to the art of femoral prosthetics, and more particularly to an improved femoral head-neck prosthesis and a method of implanting such a prosthesis.
Changes in the distribution of stress through a femur (thigh bone) after implanting a prosthesis may cause a number of complications, such as further deterioration of the bone. According to Wolff's law, changes in stress distribution through a bone eventually cause definite alterations in its internal structure. For example, if a portion of the femur is shielded from stresses that would normally occur, that portion is likely to deteriorate. On the other hand, where portions of the femur are subjected to greater stresses, those portions are likely to thicken in response. Of course, if the stresses are increased excessively bone cells will probably be killed ("necrosis").
Conventional "intramedullary" femoral head-neck implants, i.e., implants having a long stem secured in the medullary (marrow) canal of the femur, may cause deterioration of the femur since the proximal (upper) end of the femur is shielded from normal stress. As a result, an intramedullary prosthesis has a relatively short expected lifespan, at least relative to the expected lifespan of younger patients. Moreover, patients having intramedullary implants must reduce their activity substantially.
One approach to these problems is the "extramedullary" prosthetic joint disclosed in A. Huggler, U.S. Pat. No. 4,129,903. This prosthesis is also discussed in A. Huggler and H. Jacob, The Uncemented Thrust-Plate Hip Prosthesis, and A. Schreiber, H. Jacob, Y. Suezawa and A. Huggler, First Results with the "Thrust Plate" Total Hip Prosthesis, both in The Cementless Fixation of Hip Endoprosthesis 125-132 (E. Morscher ed. 1984) (hereinafter Thrust-Plate Prosthesis and First Results, respectively). The Huggler prosthesis includes a tension or tie rod through the bone, a pressure disc in contact with the femoral neck and a counter plate at the lateral side of the femur. One of the advantages of this kind of prosthesis is that there is sufficient supporting bone for an intramedullary implant if it becomes necessary to replace it.
There are, however, a number of undesirable side effects due to the Huggler prosthesis. For example, when the femur is loaded and unloaded as occurs during walking, the tension rod tends to move slightly with respect to the bone ("micromotion"), essentially because the modulus of elasticity of the bone and tie rod are different. As a result, the tie rod is almost constantly wearing at the bone and counter plate, possibly leading to fracture of the tie rod at its interface with the counter plate.
Huggler's approach includes positioning the thrust plate perpendicular to the longitudinal axis of the femoral neck and the tie rod along the longitudinal axis of the femoral neck (e.g., approximately 35 degrees from horizontal). According to Huggler, positioning the tie rod along the central longitudinal axis of the femoral neck is desirable to prevent motion of the tie rod relative to the bone and counter plate. More specifically, Huggler's position is that the more vertical the tie rod, the more the distal end of the tie rod will move within the counter plate (i.e., the greater the "micromotion").
However, since the tie rod of the Huggler prosthesis is aligned with the central longitudinal axis of the femoral neck, the tie rod is not aligned (or near alignment) with the generally vertical load on the femur caused by normal activity, such as walking. Thus, when the femur is loaded, the tie rod is subjected to a bending moment, which may lead to its fracture. This may have been a contributing cause to the tie rod fracture discussed in Thrust-Plate Prosthesis, at p. 127.
In addition, the Huggler prosthesis is anchored relatively high (proximal) on the lateral side of the femoral shaft where the cortical bone is relatively thin. This may be the reason that one-third of the patients treated with the prosthesis complained of pain in the first 6-8 months (Final Results, at p. 130), the pain apparently abating when the cortical bone has thickened sufficiently according to Wolff's law. Another cause of this pain may be the combination of the high position of the counter plate with its relatively high profile, which may cause irritation of muscles and tendons.